This invention relates to improvements in optical torque sensors especially, but not exclusively, for incorporation in power-assisted steering systems for vehicles.
In our European patent application No. 93300792.4, a torque sensor is described which comprises an input member, an output member, emitter means, first receiver means adapted to receive a first signal, second receiver means adapted to receive a second signal, and signal processing means, in which said signal processing means is adapted to receive output signals from the first and second receiver means, the output signals of the receiver means being dependent upon the first and second signals which they receive, and the signal processing means being adapted to process the output signals from the first and second receiver means so as to produce a modified output signal indicative of the relative angular displacement between, or torque applied between, the input and output member. A torque sensor of this type will hence forth be referred to as a torque sensor of the kind described.
A torque sensor of the kind described may be incorporated in a power-assisted steering system having an input column transmitting torque from a steering wheel to the input member of the torque sensor, and an output column transferring torque to a steering mechanism adapted to steer road wheels of the system. The modified output signal of the torque sensor may be used to control the operation of an electric steering motor adapted to steer the road wheels.
A highly desirable feature of any torque sensor is the facility for easy detection of partial or total failure of the sensor. This is vitally important in a safety critical application such as a vehicle steering system. If the sensor can be tested each time the system is switched on or off, or preferably continuously during operation, this allows a significant increase in the safety of the system, since the driver can be warned of the fault and the system returned to a safe state. An example of such a safe state would be to disable the power assistance in the steering system until the sensor has been replaced or repaired.
An aim of the present invention is to provide diagnostic means capable of detecting partial or total failure of a torque sensor of the kind described.